Project Summary/Abstract: Scores of people are being exposed to drug combinations that include methylphenidate (Ritalin) plus a selective serotonin reuptake inhibitor (SSRI). Such drug combinations are indicated for attention-deficit hyperactivity disorder (ADHD)/depression or anxiety comorbidity (in up to 40% of pediatric ADHD) and other psychiatric disorders. High-dose co-exposure also occurs, for example, in patients on SSRIs who abuse methylphenidate as a ?cognitive enhancer? or party drug. Many of these subjects are children or adolescents. This is a public health concern because of potential long-term behavioral and neuronal changes induced by these psychotropic drugs. Methylphenidate (a dopamine/norepinephrine reuptake inhibitor), given alone, mimics some, but not other, molecular effects of cocaine. These effects are less robust presumably because methylphenidate does not affect serotonin systems. Indeed, research in the applicants' laboratories shows that treatment with an SSRI in conjunction with methylphenidate potentiates abuse/addiction-related behavior and gene regulation, thus producing ?cocaine-like? behavioral and molecular profiles. It is unknown which serotonin receptor subtype(s) mediate these SSRI (serotonin) effects. The long-term goal of this project is to better understand how serotonin and dopamine interact to induce their effects on drug addiction-related behavior and neuronal processes. The objective of this application is to determine serotonin receptor subtypes and cell types that mediate the effects of methylphenidate plus SSRI combinations on behavioral responses to cocaine and gene regulation in striatum and nucleus accumbens in adolescent male and female rats. The central hypothesis is that SSRIs potentiate methylphenidate effects on both cocaine-induced behavior and gene regulation via activation of specific serotonin receptor subtypes. This hypothesis is based on preliminary data from the applicants' laboratories. The rationale for the proposed work is the necessity to determine the involved serotonin receptor subtypes and neuron populations, in order to lay the groundwork for identifying novel pharmacological interventions that avoid these effects. The central hypothesis will be tested by pursuing three specific aims: 1) Determine behavioral and neuronal consequences of combined methylphenidate+SSRI (fluoxetine) treatment, comparing clinically relevant doses and ?abuse doses?; 2) Determine serotonin receptor subtypes that contribute to the methylphenidate+SSRI effects on behavior and gene regulation; 3) Determine brain sites and cell populations that mediate these behavioral and molecular changes. Effects on behavior will be determined using the cocaine-induced place preference conditioning (CPP) and sensitization models. Effects on gene regulation will be assessed by in situ hybridization, RT-qPCR, and Western blot analyses. These techniques are well-established in our labs. The proposed research is significant, because elucidating the mechanisms by which serotonin potentiates psychostimulant-induced behavioral and neuronal changes will inform the future development of novel antidepressants that avoid these potential health risks.